The present invention relates generally to devices used in the growth of semiconductor material by the process of liquid phase epitaxy, and, more particularly, to slider/stator devices used in the growth of mercury cadmium telluride (HgCdTe).
HgCdTe is a semiconductor material similar to silicon, germanium and gallium arsenide. It is extremely useful as a sensing material in thermal imaging systems. One distinguishing feature of HgCdTe is that a slight variation of the relative proportions of mercury and cadmium changes the spectral sensitivity of the material. Although this can be a great advantage in designing infrared detectors, it means that the quantity of impurities in the material must be strictly controlled, since they may also alter the electrical properties of the material.
Liquid phase epitaxy (LPE) is a common method for growing high purity HgCdTe. In LPE, a molten solution of mercury and cadmium dissolved in tellurium is placed in contact with a cadmium telluride (CdTe) crystalline substrate. By reducing the temperature, mercury, cadmium and tellurium are forced to precipitate, forming a thin film HgCdTe crystal on the CdTe substrate. One of the most common techniques for growing LPE HgCdTe uses a sliding assembly to bring the molten growth solution in contact with the CdTe substrate.
In the slider technique, a substrate is placed in a recessed well in the top surface of the stator, or base, of the device, such that the surfaces of the stator and substrate are substantially co-planar. The slider, which is positioned on the stator, includes a well which holds the molten growth solution. Typically, this well extends through the slider so that the growth material may be loaded through the top of the slider. In the proper configuration for film growth, the slider is positioned so that the bottom of the growth solution well is in close proximity with, but does not touch, the substrate. The entire assembly is heated until the growth material is at the required temperature, and then the slider is moved so that the well holding the growth solution is positioned above the substrate. When the temperature is dropped, the HgCdTe precipitates as a thin film on the substrate. When the crystal growth is complete, the slider is retracted so that the growth solution is no longer in contact with the substrate and the thin film, and the entire assembly is cooled to room temperature. Although the technique appears uncomplicated, there are several problems which reduce the quality of the thin film grown. In the past, both slider and stator have been made of graphite, a refractory material which is both chemically nonreactive and easily machinable. Typically, the graphite which is used has been treated, impregnated with a hydrocarbon pitch and cured to reduce porosity and improve mechanical properties. It is then heated to a high temperature to drive off impurities and improve electrical properties.
Despite these improvements, the design and composition of the slider still cause degradation in the film quality and yield. First, the graphite surfaces of the slider flake and crumble into the growth solution. These particles adhere to the film and reduce performance. Second, impurities in the graphite tend to leach into the growth solution at high temperatures, resulting in contamination which alters the electrical properties of the material. Finally, because the tellurium-rich melt solution wets graphite, it tends to seep into any gaps between the stator and substrate, effectively welding the finished thin films and substrate to the stator.
Excessive film growth at the interface of the substrate and the growth well interface has been noticed in LPE growth of gallium aluminum arsenide using the slider technique. This growth is the result of thermally induced convection currents in the growth solution which are produced when the slider assembly is cooled to force film precipitation. This problem has been solved by lining the growth solution well with an insulating material, for example, sapphire, which reduces convection within the melt solution. For example, see the paper by Tamargo, M. C. and Reynolds, C. L., entitled "Use of Sapphire Liners to Eliminate Edge Growth in LPE (Al,Ga)As, Journal of Crystal Growth, Vol. 55, pp. 325-329 (1981), published by North-Holland Publishing Company. Although excessive edge growth does not occur in the growth of LPE HgCdTe, it has been found by the present invention that sapphire can be used in the design of the slider and stator to eliminate the design and contamination problems described above.
It is accordingly a primary object of the present invention to provide an improved apparatus for the growth of thin film semiconductors, particularly HgCdTe, by use of a slider/stator technique.